Porous materials are capable of absorbing low frequency sounds, but in many applications the depth of absorber needed to absorb low frequencies is not feasible. Porous absorption works by impeding the air particle movement of the sound wave. A porous absorber is most effective in the area of the wave with maximum particle velocity. If the absorber is against a wall, the superposition of the incident and reflected waves creates a region of maximum particle velocity at wavelength/4. For a 1kHz wave, that is about 8cm, easily manageable. Also, within a given depth of porous material, multiple high frequency wavelengths can occur, hence providing more opportunities for absorption. For a 63 Hz wave, a quarter wavelength is about 1.4 meters. So, you could hang the absorber 1.4m from the wall, or use 1.4m of absorber. Both are pretty impractical in most cases. Low frequencies can be difficult to manage in any case, but you may want to look into resonant or panel absorbers.
Philip answered very well, this is a common feature of porous foams for acoustic insulation.
Alain
The porous absorber based on fibrous material is not sufficient to tackle the gigantic wave length of lower frequency sound wave. The thickness of such porous structure is not enough to dissipate the sound energy owing to friction loss during the passage of sound energy along thickness direction of the absorber. The material appears to be a very small hurdle in front of the gigantic wave.
Porous materials are not efficient at low frequency since low frequency characterizes by long wavelength. So, you need a thick layer of acoustic material to attenuate low-frequency sound which is considered impractical.
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